The main contents in this paper are as follows:1. The geographical distribution of fluorine poisoning affected regions.2. The geographical factors in the formation of higher fluorine content water.3. Improving the environmental quality to prevent endemic fluorine poisoning. The Yu-lin Prefecture is a major affected region of Shau-Xi Province. 38.7% ofthe total population have fluorotic mottled teeth and 2.1% have fluorotic osteopathy. This is a slightly and moderately affected region.In this Prefecture, when the fluorotic content reaches or exceeds 1.0 ppm in the drinking water, the clinic symptoms and signs becomes apparent. As the average con-tent of fluorine in drinking water exceeds the national hygienical standard in greater part of the Prefecture, one may say that this is an affected region. Furthermore, it has also the conditions for the disease to reappeared in a region previously affected. The concentration of fluorine in groundwater is higher in the western and southern counties; these are the major areas of endemic fluorine poisoning in the Prefecture. The symptom, however, is scattering in the region rather than concentrated.The major geographical factors causing higher-fluroine content in water in locality are: (1) The climate; (2) The composition of surface materials: (3) The topog-raphy; (4) Biological and human- social factors.According to our study, the following measures are very important to improve the environmental quality, as well as to prevent endemic fluorine.(1) Planting vegetation (both trees and grasses), especially those fluorine-tole-rant vegetations and those having the ability to absorb fluorites.(2) Preventing and harnessing soils from being salinized.(31 "Rational distribution of the residential areas and improving the quality of their drinking water.(4) Taking measures to prevent pollution of the environments around the sources of water supply.